Inosine monophosphate dehydrogenase (IMPDH, E.C. I .1.1.205) is an enzyme whose function is closely linked with the control of cell proliferation and differentiation. It has long been recognized as an important target in the design of both anti-tumor, immunosuppressive and antiviral drugs. This project will use X-ray crystallography to provide the first structures of chemotherapeutically active dinucleotide analogues bound to human IMPDH. This information will be correlated with biochemical and biological data. The goal is the identification of specific structural features required for binding. The long-range goal is the design of new agents with improved activity and clinical efficacy. IMPDH catalyzes the committed step in the de novo synthesis of the guanine nucleotides. Inhibition of IMPDH compromises the ability of G proteins to act as transducers of intracellular signals. Inhibition results in reductions in nucleic acid synthesis, oncogene expression, and, ultimately, cell proliferation and differentiation. Two isoforms of IMPDH have been identified, labeled type I and type II. Type I is constitutively expressed in normal cells. Expression and activity of type II is dramatically up-regulated in neoplastic and other rapidly dividing cells. This isoform has been a primary target in drug design. Tiazofurin is an antitumor agent that functions by inhibiting IMPDH. In Phase II clinical trials, tiazofurin has produced complete hematologic remissions in patients with end-stage acute leukemias. Tiazofurin is a prodrug. In vivo, it is converted to an analogue of the cofactor nicotinamide adenine dinucleotide (NAD). This NAD analogue, called TAD (thiazole-4-carboxamide adenine dinucleotide) is the major inhibitor of IMPDH. The selenium analogue of TAD, called SAD, binds IMPDH with equal efficacy. The principal investigator has obtained a structure of a complex between the human type II isoform of IMPDH and the dinucleotide inhibitor SAD. These data demonstrate the location and conformation of the bound inhibitor, and indicate specific interactions which can be exploited in the development of compounds with improved affinity and specificity. The principal investigator will examine type I and II IMPDH binding by agents designed to 1) test the hypothesis that intramolecular constraints enhance binding and 2) enhance specificity for the type II isoform and overcome clinical problems associated with drug resistance and rapid metabolism.